An internal combustion engine mounted in a mobile machine, such as an automobile, a motorcycle, and so forth, is required to achieve reduction in weight in order to implement higher efficiency in energy consumption. There has been known a valve spring retainer, as a constituent member of a valve mechanism of an internal combustion engine, that is made of titanium which is a lightweight and high-strength material, in an attempt to achieve reduction in the weight of the valve mechanism, leading to reduction in the weight of the internal combustion engine (JP-A No. 240639/1989). In this case, the valve spring retainer is formed by using a β-titanium alloy.
Incidentally, titanium material includes a pure titanium material superior in workability, but low in strength, a titanium alloy, and α-β titanium alloy, both of which are inferior in workability, but high in strength at high temperature, and β-titanium alloy that is suited for cold plastic working, and can attain high strength by applying heat treatment thereto.
In the case of using the pure titanium material, although bolts have been formed by cold forging in a limited application, such an application is limited to JIS class 1 material that is low in strength. The JIS class 1 material is mainly used to obtain corrosion resistance, and with the JIS class 1 material, it is impossible to obtain a specific strength expected of titanium.
For machinery components such as internal combustion engine components, and so forth, the α-β titanium alloy, such as Ti-6Al-4V alloy, and so forth, is in widespread use from a strength point of view. When the α-β titanium alloy is used, however, forming thereof is carried out by hot forging at high temperature, so that a great deal of after-working is required owing to problems of oxidation of the surface thereof, and precision in size. As a result, not only material cost, but also processing cost increases, so that titanium components become very expensive, resulting in difficulty with application thereof to common valve spring retainers for internal combustion engines for vehicles.
Further, the β-titanium alloy can be cold rolled in the process of preparing a raw material, however, when forming components thereof by cold forging, there is a problem with mass production in terms of service life of a mold because of high deformation strength of the raw material, and further, an addition element required for stabilizing a β-phase is expensive, and is high in specific gravity, so that the advantageous effect of reduction in cost cannot be gained.
In view of such circumstances as described, the invention has been developed, and it is an object of the invention to provide a valve spring retainer made of titanium, capable of achieving reduction in material cost and processing cost.